1. Field of the Invention
The present invention relates to a drive apparatus for an electromagnetic actuator. More particularly, the present invention relates to a PWM drive apparatus which has a small size and light weight achieved by semiconductor circuit technology and is capable of driving an actuator with a high level of power efficiency.
2. Description of the Related Art
In recent years, a PWM drive method is commonly used as a means for reducing power consumption required to drive electromagnetic actuators for focus, tracking and tilt controls, a spindle motor, a pickup moving motor or the like in optical disc players. The size and weight of a PWM drive apparatus are reduced by packing major parts thereof into a single semiconductor chip.
FIG. 11 shows a structure of a conventional PWM drive apparatus. A lag-lead filter comprising resistances 211 and 212 and a capacitance 213 which are connected in series is connected to an input terminal 101 of a voltage Vin. The voltage Vin is a command voltage which commands an average drive voltage to be applied to a load 200. An absolute value circuit 214 receives a voltage at a connection point of the resistances 211 and 212, and outputs the absolute value Vabs and the sign Vsign of the voltage. A PWM modulator 215 PWM-modulates the absolute value Vabs using a triangular wave which is generated by a triangular wave oscillator 216. A drive section 217 drives the load 200 by supplying a voltage to a positive output terminal 102 and a negative output terminal 103 based on a PWM-modulated signal and the sign Vsign which is input through a direction switch circuit 218. A voltage VFO of the positive output terminal 102 and a voltage VRO of the negative output terminal 103 are input to a differential-input voltage-to-current converter 219, and a current proportional to the difference voltage is supplied to the capacitance 213. Thus, the lag-lead filter comprising the resistances 211 and 212 and the capacitance 213, and the voltage-to-current converter 219 form a negative feedback circuit in the PWM drive apparatus.
FIG. 12 equivalently shows the PWM drive apparatus of FIG. 11 using two blocks, i.e., a gain block having a gain G which includes the absolute value circuit 214, the drive section 217 and the load 200, and a modulation index block having a modulation index M (0≦M≦1) which includes the triangular wave oscillator 216 and the PWM modulator 215. A closed loop gain Gclose of the PWM drive apparatus is represented by:
            G      close        ⁢          ❘              Rb        =                  1                      GM            ·            gm                                =            1                        gm          ·          Ra                +                  1          GM                      ≈                  1                  gm          ·          Ra                    ⁢              (                  GM          >>          1                )            where Ra represents the resistance value of the resistance 211, Rb represents the resistance value of the resistance 212, and gm represents the transconductance of the voltage-to-current converter 219.
Specifically, the closed loop gain of the PWM drive apparatus is approximately determined based on the resistance value of the resistance 211 and the transconductance of the voltage-to-current converter 219. In general, however, a transconductance is determined as an operation of a semiconductor circuit, while a resistance value is determined based on the dimension and material of a resistance, so that a gain change occurs due to a device-to-device variation, a temperature change, or the like.
Further, when the command voltage Vin which is zero is input, an offset voltage is applied to the load 200 due to an offset current of the voltage-to-current converter 219, so that an incorrect torque occurs in an electromagnetic actuator or the like. Also, saturation occurs at either a positive or negative peak, depending on the magnitude of the command voltage Vin, so that a distortion occurs in an output current and a voltage waveform.
To avoid the above-described drawback, a semiconductor element included in the voltage-to-current converter 219, particularly an element involved in a differential operation or the like, is caused to be as large as possible so as to reduce variations in electrical resistance due to an error in dimension of the semiconductor element during formation, thereby making it possible to reduce variations in voltage and current in the circuit. However, this solution runs counter to the reduction of power consumption by recent microfabrication or cost reduction by decreasing the area of a chip, i.e., cost increases due to increases in power consumption and chip area.
It is also possible to adjust a voltage and a current into predetermined values by changing a circuit constant by laser trimming or the like. However, this solution requires a trimming process after measurement of an offset current or the like for each individual device, so that considerable effort and time are required, unavoidably leading to an increase in cost.